The key line in /etc/ostree/remotes.d/fedora-workstation.conf in Fedora 29 should be changed to gpgkeypath=/etc/pki/rpm-gpg/ so rebasing to the upcoming Fedora 30 works.
/etc/ostree/remotes.d/fedora-workstation.conf
gpgkeypath=/etc/pki/rpm-gpg/
This will be fixed by https://pagure.io/workstation-ostree-config/pull-request/143. It introduces a new fedora remote that contains the optimal settings.
fedora
Once that PR merges, you should be able to update and then rebase fedora:.... Though you can of course add the remote manually today too (see https://src.fedoraproject.org/rpms/fedora-repos/pull-request/33#request_diff).
rebase fedora:...
Metadata Update from @tpopela: - Issue status updated to: Closed (was: Open)